The demand for higher performance materials with optimum combinations of properties is steadily becoming more critical. For steels, the microstructure controls the resulting mechanical properties and hence, the desired property profile requires the development of a properly adjusted microstructure. The traditional way of producing a fine-grained microstructure yielding the optimum combination of strength and toughness is through thermomechanical processing. By such processing, an effective ferrite grain size well below 5 μm can readily be achieved, even in thick steel plates. In addition, the use of advanced ladle refining techniques for deoxidation and desulphurisation has lead to further quality improvements through a general reduction in the steel oxygen and sulphur contents. The impurity level reflects the amount of non-metallic inclusions being bound as oxides and sulphides in the steel. The harmful effect of inclusions on steel properties arises from their ability to act as initiation sites for micro-voids and cleavage cracks during service. Hence, the use of clean steels is normally considered to be an advantage from a toughness point of view.
Inclusions do not always cause a problem in steel. The catalytic effect of the inclusions on the microstructure evolution can be exploited, both during solidification and in the solid state, by virtue of their ability to act as potent heterogeneous nucleation sites for different types of transformation products such as ferrite and austenite. In this case the key issue is to control the inclusion size distribution during the manufacturing stage, which is a major challenge. Therefore, a successful result is contingent upon that both the maximum and minimum diameters as well as the mean size of the inclusions in the as-cast steel can be kept within very narrow (specified) limits.
This is due to two conflicting requirements. On the one hand, a submicron particle size below, say, 0.2 to 0.4 μm implies that the inclusions start to lose their nucleation potency because a curved interface increases the associated energy barrier against heterogeneous nucleation. On the other hand, if the inclusion size is significantly larger than 2 to 4 μm they become detrimental to toughness. At the same time the number density drops rapidly, which, in turn, increases the grain size in the finished steel. Under such conditions the latent grain refining potential in the steel is reduced to an extent which makes grain refinement by inclusions impossible from a transformation kinetic point of view.
In order to promote grain refinement by active inclusions in steels, two possible routes can be followed. The conventional route, which has been extensively explored in the past, is to create the nucleating inclusions within the system during steelmaking by modifying the applied deoxidation and desulphurisation practice. This has lead to the development of new steel grades, where a significant part of the grain refinement is achieved through heterogeneous nucleation of ferrite or austenite at active inclusions following cooling through the different transformation ranges. Unfortunately, uncontrolled coarsening of the inclusions in the liquid steel prior to solidification is still a major problem during industrial steelmaking, meaning that these new steel grades have not found a wide application. However, by following a new route and utilising specially designed grain refiners containing a fine distribution of the nucleating particles (which then are added to the liquid steel before the casting operation), improved conditions for grain refinement can be achieved during subsequent steel processing, without compromising the toughness. This is a well-proven technology in casting of aluminium alloys, which later has been transferred to the ferrous sector. Provided that the resulting particle number density and volume fraction are of the correct order of magnitude, the use of such grain refiner can enable full-scale production of new steel grades, provided that they do not have a negative influence on the steelmaking process itself. WO 01/57280 describes a grain refinement alloy for steel containing between 0.001 and 2% by weight of oxygen or sulphur. Note that term alloy in this context means a metal-based grain refiner always being low in the non-metallic elements O and S.
However, in grain refinement of steel oxygen and sulphur are the key elements controlling the particle volume fraction and number density of the nucleating inclusions in the as-cast product. Thus, in order to achieve the desired degree of grain refinement during subsequent steel processing, the grain refining alloy described in WO 01/57280 must be added in amounts that, at least, exceed one percent by weight of the liquid steel melt. This level of addition is not acceptable in continuous casting of steels, where the maximum limit is typically 0.2 to 0.3% by weight of the liquid steel to avoid problems related to the dissolution and mixing of the grain refining alloy in the tundish or the casting mould. Addition of larger amounts (>0.5 wt %) of cold alloy in liquid steel will also cool the steel to an extent that it starts to freeze in the inlet die of the casting mould, thereby destroying the casting operation.
A breakthrough in the existing grain refinement technology is therefore required to fully exploit the potentials of the concept in industrial steelmaking. The object of the present invention is to transfer the technology to continuous casting of steels, which is the dominating casting method for wrought steel products, covering more than 90% of the world wide steel production.